def send_message(sender, recipient, message_body, priority=1):
    """Send a message to a recipient

    :param str sender: The person sending the message
    :param str recipient: The recipient of the message
    :param str message_body: The body of the message
    :param priority: The priority of the message, can be a number 1-5
    :type priority: integer or None
    :return: the message id
    :rtype: int
    :raises ValueError: if the message_body exceeds 160 characters
    :raises TypeError: if the message_body is not a basestring
    """
    # Function definition